Extratropical Highlights –November 2018

 

1. Northern Hemisphere

The 500-hPa circulation during November featured above-average heights over Alaska, western Canada, Greenland and Scandinavia. Below-average heights were present from central North America to western Europe, and over central Russia (Fig. E9). This overall anomaly pattern projected strongly onto the positive phases of the East Atlantic/ Western Russia (+1.5 std. dev.) and Scandinavia (+1.9 std. dev.) teleconnection patterns (Fig. E7, Table E1). Notably, November was the first month since March, and only the second month since January, without a strong positive phase of the North Atlantic Oscillation (NAO).

The main land-surface temperature signals during November included above-average temperatures in Alaska, Europe and northeastern Asia, and below-average temperatures in the eastern half of North America (Fig. E1). The main precipitation signals included above-average totals across the eastern U.S., and below-average totals in northern Europe (Fig. E3).

 

a. United States

The 500-hPa circulation during November featured an amplified wave pattern across North America, with above-average heights over Alaska and below-average heights over the central U.S. and Canada (Fig. E9). This pattern brought a continuation of exceptionally warm surface temperatures to Alaska, and below-average temperatures to central North America (Fig. E1). In Alaska, many areas again recorded departures of 3+ºC and in the upper 70^th percentile of occurrences. In contrast, temperatures across central North America were generally 1-2ºC below average and in the lowest 30^th percentile of occurrences. The amplified trough also contributed to well above-average precipitation across the eastern U.S. (Fig. E3). The most significant surpluses (in the upper 90^th percentile of occurrences) were observed in the Ohio Valley, Northeast, Mid-Atlantic, Southeast, and Gulf Coast regions (Fig. E5).

According to the U.S. Drought Monitor, severe-to-exceptional drought continued in the four-corner states of Utah, Colorado, Arizona, and New Mexico. Farther north in Oregon, extreme drought persisted across the central part of the state, and severe drought persisted elsewhere.

 

b. North Atlantic/ Europe

The 500-hPa height pattern featured above-average heights over Scandinavia and the high latitudes of the North Atlantic, and below-average heights over the central North Atlantic (Fig. E9). This pattern projected strongly onto the positive phases of the East Atlantic/ Western Russia (+1.5 std. dev.) and Scandinavia (+1.9 std. dev.) teleconnection patterns (Fig. E7, Table E1). The overall circulation contributed to a continuation of exceptionally warm surface temperatures across Europe and Scandinavia (Fig. E1), with most areas again recording departures in the upper 70^th percentile of occurrences. It also contributed to a continuation of below-average precipitation in northern Europe, an area which has experienced rainfall deficits for the last seven months (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during November featured above-average heights over the central Indian Ocean and in the area south of New Zealand, and below-average heights over the high latitudes of the eastern South Pacific and in the area south of Australia (Fig. E15). The most significant precipitation anomalies during November reflected a continuation of above-average precipitation in central South America, and well below-average totals in the South African monsoon region (Fig. E3).

The South African monsoon season runs from October to April. So far in 2018, this area recorded well below-average precipitation during October and November, with many locations during November recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

The Antarctic ozone hole typically develops during August and reaches its peak size in September. The ozone hole then gradually decreases during October and November, and dissipates in early December (Fig. S8, top). During November 2018, the size of the ozone hole was exceptionally large (near 18 million square kilometers) at the start of the month, and had completely dissipated by the end of the month. Overall for 2018, the ozone hole was larger than average during mid-September through mid-November, in association with an above-average amount of polar stratospheric cloud (PSC, Fig. S8 bottom) and an expanded polar vortex (Fig. S8 middle).